I know that they do this with the really big (underground) petrol tanks in petrol stations. They use an epoxy based paint. I guess you could do the same with a small tank, but painting the inside of a small tank might be difficult.Considering the cost of a new tank and the potential damage caused by a leaking old tank I am not sure this would be worth the risk.

Juggling active hand grenades (Croatia, 2001),Leaving a lit cigarette in a warehouse full of explosives Jumping out of a plane to film skydivers without wearing a parachute (U.S., 1987), Trying to get enough light to look down the barrel of a loaded muzzleloader gun using a cigarette lighter (U.S., 1996), Using a lighter to illuminate a fuel tank to make sure it contains nothing flammable (Brazil, 2003), Attempting to play Russian roulette with a semi-automatic pistol that automatically reloads the next round into the chamber, or attempting Russian roulette with an unexploded landmine, Crashing through a window and falling to your death in trying to demonstrate that the window is unbreakable,